1. Field of the Invention
This invention relates to a driving device for a stepping motor.
2. Related Background Art
A constant current chopper driving circuit of good efficiency has heretofore often been used as a driving circuit for a stepping motor. An example of it will hereinafter be described with reference to FIG. 6 of the accompanying drawings.
FIG. 6 shows an example of a driving circuit for a 4-phase stepping motor SM, and in view of the fact that a driving circuit on I and III phase coils CL1 and CL3 side is entirely the same as a driving circuit on II and IV phase coils CL2 and CL4 side, this figure shows only the circuit arrangement on the I and III phase side.
In FIG. 6, Tr1 and Tr2 designate phase current chopper transistors for the I phase coil CL1 and the III phase coil CL3 of the stepping motor SM. Tr3 and Tr4 denote transistors which are supplied with phase excitation signals .phi.1 and .phi.3 for the I and III phase coils CL1 and CL3 and control the ON and OFF of the excitation current for these coils CL1 and CL3.
D1 and D2 designate diodes for preventing the backward flow of the excitation current, and D3 denotes a diode for feeding back the current when the chopper transistor Tr1 is turned off, as will be described later. Diodes D4 and D5 and Zener diode ZD1 are diodes for protecting the transistors Tr3 and Tr4 and a diode D6 is a diode for protecting the transistor Tr1.
R1-R7 designate resistors. The detection voltage of a phase current detecting resistor R.sub.S1 connected between the common emitter of the transistors Tr3 and Tr4 and a common potential is applied to the negative input terminal of a comparator IC1 through the resistor R5. An output reference voltage E from a reference voltage generator is applied to the other positive input terminal of the comparator IC1 through the resistor R7. The resistor R6 is connected between this positive input terminal and the output terminal of the comparator.
Further, the output terminal of the comparator is connected to the base of the transistor Tr2 and the resistor R2, and the collector of the transistor Tr2 is connected to the base of the transistor Tr1 through the resistor R1.
The reference voltage generator 1 comprises a D/A digital to analog converter, and D/A-converts the reference voltage indication by a digital signal from a microprocessor, not shown, and outputs the reference voltage E.
Vcc denotes a power source voltage.
Assuming that of the phase excitation signals .phi.1 and .phi.3 corresponding to the I phase and the III phase, the signal .phi.1 has come, the transistor Tr3 is biased thereby through the resistor R3 and conducts, and a phase current flows through the route of Tr1.fwdarw.CL1.fwdarw.D1.fwdarw.Tr3.fwdarw.R.sub.S1. This phase current rises at a certain time constant by the inductance load of the I phase coil CL1.
When the potential across the detecting resistor R.sub.S1 produced by this phase current reaches the reference voltage E, the output of the comparator IC1 assumes a low level, and the transistor Tr2 so far biased by the resistor R2 becomes non-conductive. As a result, the transistor Tr1 also becomes nonconductive.
At this time, the phase current flows through the route of D3.fwdarw.CL1.fwdarw.D1-Tr3.fwdarw.R.sub.S1 by energy stored in the I phase coil CL1, but as the energy of the I phase coil CL1 is consumed, the phase current decreases and the potential across the detecting resistor R.sub.S1 also falls. When the potential across the detecting resistor R.sub.S1 falls below the reference voltage E, the output of the comparator IC1 becomes a high impedance, and the transistors Tr2 and Tr1 conduct again and the phase current flow through the route of Tr1.fwdarw.CL1.fwdarw.D1.fwdarw.Tr3.fwdarw.R.sub.S1.
By the repetition of the above-described process, the phase current i.sub.1 of I phase is chopped by the transistors Tr1 and Tr2 during the period of production of the I phase excitation signal .phi.1 and becomes a constant current i.sub..phi.1 =E/R.sub.S1. This state is shown in FIG. 7 of the accompanying drawings.
Actually, the circuit shown in FIG. 6 has a hysteresis determined by the resistors R6 and R7 and a delay of a feedback loop and therefore, the I phase current i.sub..phi.1 becomes a "sawtooth wave" as shown in FIG. 7. The period indicated by t.sub.1 in FIG. 7 is a period during which the energy stored in the I phase coil CL1 after the transistor Tr3 has become non-conductive flows through the route of CL1.fwdarw.D1.fwdarw.D4.fwdarw.ZD1.
However, in the example of the prior art shown in FIGS. 6 and 7, the rising period t.sub.0 (see FIG. 7) of the I phase current i.sub.1 is ##EQU1## (where R.sub.L1 and L.sub.1 are indicative of the winding resistance and inductance, respectively, of the I phase coil CL1), and the manner of rising of the I phase current i.sub..phi.1 depends on the power source voltage Vcc.
As a result, in a system wherein the rising of the phase current i becomes steep due to the relation between the power source voltage Vcc and the winding resistance and inductance of the stepping motor SM, there has been the disadvantage that the ripple of the torque produced during the rotation of the stepping motor SM, particularly during the change-over of the phase, becomes great and the noise produced from the stepping motor SM becomes great.
Also, in the aforedescribed example of the prior art, the switching period T of the chopper transistor Tr1 shown in FIG. 6 becomes great depending on the system, and this also has led to the disadvantage that the chopper frequency (1/T) enters into the audible range to cause discordant noise.